Those fears are likely to be raised during talks in Dublin between the top Russian and US diplomats and the international peace envoy, with the UN chief saying Syrian President Bashar al-Assad must face justice if he uses his chemical arsenal.

Assad forces shelled Douma and Zabadani to the northeast of the capital and Daraya and Moadamiyet al-Sham to the southwest, official and activist sources said.

As the army escalated its assault on rebel-held Daraya, the scene in August of the single worst massacre in Syria's 21-month conflict, additional troops were deployed to the town.

"Syrian army units continued today to pursue terrorists loyal to (the radical Islamist) Al-Nusra Front, which is part of al-Qaida, in Daraya," state news agency SANA said.

A rebel fighter runs for cover during heavy clashes with government forces in Aleppo.

Citing an unnamed military source, the agency also said "Daraya will be completely cleansed of terrorists soon," using its standard term for rebels.

An anti-regime activist said that the army is advancing little by little into Daraya, though the rebel Free Syrian Army is fighting hard to keep the troops out of the town.

"The army has been able to enter into around 30 per cent of Daraya in recent days, and there are serious fears about what would happen to the town if the troops do reclaim it," said the man, who identified himself as Abu Kinan.

At the same time, there were clashes in Irbin to the east, the Syrian Observatory for Human Rights said.

In the capital itself, security forces swarmed the southern district of Zahra after a car bomb exploded, with the Observatory saying no one was killed.

But state television said "Al-Qaida terrorists exploded a bomb in a car in front of a Red Crescent centre in Damascus, causing one death and major damage."

Damascus province is now a key battlefield, as regime forces battle to retake control of an eight-kilometre belt around the capital, analysts say.

In the embattled northern city of Aleppo, several districts saw clashes, while shells slammed into zones in the southern province of Daraa, the Observatory said.

The Observatory said the fighting had claimed 104 lives across the country, according to a toll compiled from a network of activists, lawyers and doctors.

The Britain-based watchdog has tallied more than 41,000 deaths, most of them civilians, since the uprising erupted in March 2011.

In Dublin, US Secretary of State Hillary Clinton will meet Russian Foreign Minister Sergei Lavrov and UN peace envoy Lakhdar Brahimi amid fears the regime could be preparing to use chemical weapons as it battles the rebels.

Mrs Clinton warned Damascus on Wednesday that this was a clear red line that must not be crossed.

"Our concerns are that an increasingly desperate Assad regime might turn to chemical weapons or might lose control of them to one of the many groups that are now operating within Syria," Mrs Clinton said.

Speaking in Baghdad yesterday, UN chief Ban Ki-moon said Assad should be "brought to justice" if his regime uses chemical weapons.

Mr Ban said he had warned that whoever does so "will have to be brought to justice, and it will create serious consequences to those people."

Mrs Clinton also again pressed the Assad regime to make "the decision to participate in a political transition, ending the violence against its own people."

Moscow has been a staunch ally of Assad and along with China has vetoed UN Security Council resolutions aiming to sanction the regime.

Washington has long been calling for Moscow to use its leverage with Assad to try to persuade him to step down, and open the way towards a political transition.

Russian President Vladimir Putin met Turkish Prime Minister Recep Tayyip Erdogan in Istanbul on Monday on a trip focused on resolving sharp differences over the conflict.

Last month, Mr Erdogan said Russia held the key to the Syrian conflict, and that if Moscow took a "positive" stance in the Security Council it could push another key Damascus ally, Iran, to review its policies.

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